Proppant control in an lpg frac system

ABSTRACT

An apparatus for supplying proppant is disclosed, comprising a vessel and a level monitor. The vessel has an interior containing a mixture of proppant and liquid, an inlet for supplying proppant to the interior, and an outlet for supplying the mixture of proppant and liquid from the interior of the vessel. The level monitor is associated with the vessel for monitoring at least a level of liquid and has a discriminator for discriminating between the level of liquid and a level of proppant in the mixture of proppant and liquid. A method of supplying proppant is also disclosed. A vessel is provided at least partially filled with a mixture of proppant and liquid and having a level of liquid and a level of proppant. The level of liquid in the mixture of proppant and liquid is monitored. The mixture of proppant and liquid is supplied from the vessel through an outlet in the vessel.

TECHNICAL FIELD

This document relates to methods and apparatus for controlling proppant in a well treatment system.

BACKGROUND

In hydraulic fracturing, frac fluids may be sent to a high pressure pump to be pumped down a well to fracture a formation. Typically, these frac fluids contain proppant supplied into the frac fluid for propping open fractures created in the formation by the pressure of the frac fluid. Proppant may be supplied into the frac fluid from a proppant supply source. Fluid may be added to the proppant supply source to assist in controlling gas break out. A method of monitoring fluid level in the proppant is required.

SUMMARY

An apparatus for supplying proppant is disclosed, comprising a vessel and a level monitor. The vessel has an interior containing a mixture of proppant and liquid, an inlet for supplying proppant to the interior, and an outlet for supplying the mixture of proppant and liquid from the interior of the vessel. The level monitor is associated with the vessel for monitoring at least a level of liquid and has a discriminator for discriminating between the level of liquid and a level of proppant in the mixture of proppant and liquid.

A method of supplying proppant is also disclosed. A vessel is provided at least partially filled with a mixture of proppant and liquid and having a level of liquid and a level of proppant. The level of liquid in the mixture of proppant and liquid is monitored. The mixture of proppant and liquid is supplied from the vessel through an outlet in the vessel.

An apparatus for supplying proppant is also disclosed, comprising a pressure vessel and a level monitor. The pressure vessel has an interior containing a mixture of proppant and liquid, an inlet for supplying proppant to the interior, and an outlet at or near a base of the interior for supplying the mixture of proppant and liquid from the interior of the pressure vessel. The level monitor is associated with the pressure vessel for monitoring at least a level of liquid of the mixture of proppant and liquid.

These and other aspects of the system and method are set out in the claims, which are incorporated here by reference.

BRIEF DESCRIPTION OF THE FIGURES

Embodiments will now be described with reference to the figures, in which like reference characters denote like elements, by way of example, and in which:

FIG. 1 is side elevation view, in section and not to scale, of a proppant vessel containing a level monitoring device.

FIG. 2 is a side elevation view, in section and not to scale, of a portion of a level discriminator.

FIG. 3 is a schematic of a proppant addition system.

FIG. 4 is a side elevation view, partially in section and not to scale, of a proppant supply source contained on a truck flatbed.

FIG. 5 is a side elevation view, in section and not to scale, of a portion of a further level discriminator.

FIG. 6 is a side elevation view, in section and not to scale, of a pressure vessel and a level monitoring device.

FIG. 7 is a perspective view, not to scale, of the lower end of a discriminator.

FIG. 8 is a side elevation view, in section and not to scale, of a proppant vessel containing a level monitoring device having multiple systems.

FIG. 9 is a flow schematic of a method of supplying proppant.

DETAILED DESCRIPTION

Immaterial modifications may be made to the embodiments described here without departing from what is covered by the claims.

Proppant may be required to be supplied into a stream of fluid, for example a stream of frac fluid. In some cases it is desirable to supply the proppant as a mixture of proppant and liquid. This may wet the proppant, allowing it to be more easily transferred from the proppant supply source. In cases where the proppant supply source is under positive pressure, the liquid in the proppant can act as a liquid seal to prevent gas breakthrough into the stream of frac fluid from the proppant vessel.

Referring to FIG. 1, an apparatus 10 for supplying proppant is illustrated. Apparatus 10 comprises a vessel 12 and a level monitor, for example a level monitoring device. Vessel 12 has an interior 16 containing a mixture of proppant and liquid, an inlet 18 for supplying proppant to the interior 16, and an outlet 20 for supplying the mixture of proppant and liquid from the interior 16 of the vessel 12. Referring to FIG. 6, outlet 20 may be connected to supply proppant into a pressurized stream of fluid 24. The vessel 12 may be a positive-pressure vessel as illustrated in FIG. 6, for example if supplying proppant into a stream of pressurized fluid or if the liquid comprises liquefied petroleum gas. Referring to FIG. 1, vessel 12 may have a liquid inlet 22 for supplying liquid to the interior 16 of the vessel 12.

Referring to FIG. 1, level monitor 14 is associated with the vessel 12 for monitoring at least a level of liquid 26. Level monitor 14 has a discriminator 28 as part of level monitor 14 for discriminating between the level of liquid 26 and a level of proppant 30 in the mixture of proppant and liquid. Referring to FIGS. 4 and 1, level monitor 14 may be located at least partially outside the interior 16 of the vessel 12 (FIG. 4), or within vessel 12 (FIG. 1) for example attached to a surface of the interior 16.

In some embodiments, the mixture of proppant and liquid is a mixture of proppant wetted with liquid. In some embodiments, a suitable amount of liquid is maintained or supplied into interior 16 to provide at least one of a liquid seal and saturation of the pores of the proppant contained within. Because sand has around 30% porosity an exemplary load of 15 tonnes of sand would contain 3 m³ of propane.

Referring to FIG. 1, in some embodiments the discriminator 28 comprises a sheath 34 having an upper end 36 and a lower end 38. Sheath 34 also has an opening 40 at or near the lower end 38 for allowing liquid in the interior 16 to flow into the sheath 34. Opening 40, which may be multiple openings, at least restricts the flow of proppant into the sheath 34, and preferably prevents proppant from entering sheath 34 altogether, thus at least partially isolating the liquid from the proppant so the liquid level 26 can be measured. Sheath 34 also has an equalization vent 42 for allowing the pressure inside the interior 16 and inside the sheath 34 to equalize. Vent 42 may be a hole or perforated surface for example. In some embodiments, discriminator 28 can discriminate between the level of liquid 26 and the level of proppant 30 regardless of the relative level of either in the vessel 12.

Sheath 34 may be a radar rod, for example of the type sold by Endress and Hauser. Sheath 34 may also have a flange 35 for coupling to a head 13 of level monitor 14 for example by bolting. Referring to FIG. 4, flange 35 may be adapted to couple to at least one of both inlet 18, for example the hatch of the vessel 12, and head 13. This way, when inlet 18 is opened, sheath 34 may be easily removed or inserted as desired. Referring to FIG. 1, in some embodiments, sheath 34 is straight along a sheath length, which may be from the upper end 36 to the lower end 38. This may be necessary if the level monitor 14 operates on a line-of-sight detection basis. Referring to FIG. 1, in some embodiments sheath 34 spans from an upper end 44 of the interior 16 to a lower end 46 of the interior 16. This way, discriminator 28 may discriminate between the proppant and the liquid over the entire or substantially the entire interior height of the inside of the tank. Referring to FIG. 3, in some embodiments sheath 34 only spans a portion of the interior height of the vessel 12.

Referring to FIG. 1, opening 40 may comprise at least one slot as shown. In some embodiments, opening 40 may effectively consist of openings along a portion or the entire length of sheath 34, for maximum permeation of the liquid into the sheath 34. Referring to FIG. 7, opening 40 may comprise a proppant screen 48. Screen 48 is understood to allow liquid to pass into sheath 34, while preventing proppant from entering. In some embodiments, the entire length of sheath 34 may be a proppant screen, or a perforated surface for example.

Referring to FIG. 1, sheath 34 may be angled relative to vertical. This may be advantageous depending on the configuration of interior 16 of vessel 12. For example, in the embodiment illustrated, lower end 46 of vessel 12 feeds to an outlet 20, for example an auger. If outlet 20 is centrally located in the base of vessel 12, then there may be no room for lower end 38 of sheath 34 to extend into, and sheath 34 can be angled to the nearby side of outlet 20.

Level monitor 14 monitors the amount of liquid that is contained in the proppant supply source 12. This may allow a user to estimate the effective size of the liquid seal provided by the liquid in the mixture at the outlet of the proppant supply source. The amount of liquid may be monitored by taking at least one of direct and indirect measurements of a level of liquid. As further described below, the level of liquid may be the actual height of the liquid in the proppant supply source, or it may be other measurable levels for example the level liquid rises to in sheath 34. In order to accurately measure a level of liquid and infer the amount of liquid present, calibration of the measuring equipment may be required, to take into differences in for example proppant, liquid, container size, and outlet location. The discriminator 28 is configured to distinguish between the level of liquid and the level of proppant in the mixture, so that at least the level of the liquid can be measured.

This distinguishing may be accomplished at least in part by isolating at least one of the levels, for example by physically separating the liquid from the proppant using sheath 34. The distinguishing may also be done at least in part electronically, for example by analyzing measured data obtained from monitor 14 using a computing device. The analyzation part may be provided for example in at least one of head 13 and from a remote location for example in a control console. In this way, monitor 14 obtains measurements that are then interpreted by another part of the level monitor 14. The final information may be then conveyed to at least one of a user and a control algorithm, and may be used to make further decisions. For example, if the level of liquid, and hence the effectiveness of the liquid seal is low, a decision may be made to increase the amount of liquid in the mixture. Any software used is contained on a computer readable medium.

Referring to FIG. 2, level monitor 14 (shown in FIG. 1) may be configured to use time domain reflectometry (TDR) to determine the level of liquid 26 within the sheath 34. This type of measurement involves the use of a sonic device that emits waves 52 from for example head 13 which bounce off the level 26 and return as return waves 54, wherein determining the height of liquid in the sheath is based on the time difference between sending waves 52 and detecting waves 54. The waves may be radio waves for example. The waves may be guided by the sheath 34. A suitable system that can accomplish this is the Levelflex M FMP41C from Endress and Hauser.

Referring to FIG. 5, other methods may be used such as a float 56—operated system. Level monitor 14 may detect the position of float 56, and hence infer level 26.

Referring to FIG. 6, another example of a method of detection is illustrated using seismic detection, where level monitor 14 sends out sound waves to detect directly levels 26 and 30. This is one example of an embodiment where the level monitor is configured to monitor both the level of liquid 26 and the level of proppant 30. In this case, the discriminator comprises software in a computing device that distinguishes between the reflections from the liquid and from the proppant. This embodiment may also work if level 26 is above level 30 as shown, and level monitor 14 is a TDR device that can detect the interface between liquid and proppant (ie level 30) as well as level 26 itself. Some TDR devices can only detect the level of liquid in proppant if the level 26 of liquid is above the proppant level 30, and thus a physical discriminator 28 is required.

Referring to FIG. 8, another example of detecting both levels is illustrated, using a level monitor 14 having two systems 14A and 14B to detect levels 26 and 30, respectively. System 14A may be the same as level monitor 14 illustrated in FIG. 1, while system 14B may be a TDR device.

A further option is to use a camera such as a video camera and image analysis software. Image analysis software can detect edges in an image such as an edge caused by the levels 26 or 30. The camera can be inside the vessel 12 or directed towards a sight tube on the exterior of the vessel, though this complicates design of the vessel 12 and is not preferred. Other embodiments may detect level 26 using refractive index measurements for example with fibre optic cables, or using infra-red detectors. In other embodiments, a density meter may be used.

Referring to FIG. 6, a further apparatus 10 is illustrated for supplying proppant comprising a pressure vessel 12 and a level monitor, for example level monitor 14. Pressure vessel 12 has an interior 16 containing a mixture of proppant and liquid, an inlet 18 for supplying proppant to the interior 16, and an outlet 20 at or near a base of the interior 16 for supplying the mixture of proppant and liquid from the interior 16 of the pressure vessel 12. Level monitor 14 is associated with the pressure vessel 12 for monitoring at least a level of liquid 26 of the mixture of proppant and liquid. Referring to FIG. 1, this type of apparatus 10 is also illustrated, with level monitor 14 having a discriminator 28. This type of apparatus is useful when the interior 16 is required to be under pressure, for example when the liquid comprises liquefied petroleum gas, or when the outlet 20 supplies the mixture into a highly pressurized stream of fluid 24. Because of the positive pressure of vessel 12, it is important to monitor the level of liquid 26 in order to ensure that enough liquid remains at the base of the interior 16 to adequately provide a liquid seal. Referring to FIG. 6, the liquid seal acts provides a barrier against which pressure from the interior 16, for example provided through a gas inlet 51, may press to supply the mixture from interior 16 while preventing gas breakthrough out of outlet 20.

Referring to FIG. 9, a method of supplying proppant is illustrated. Referring to FIG. 1, in stage 100 (shown in FIG. 9) a vessel, for example vessel 12 as shown, is provided at least partially filled with a mixture of proppant and liquid and having a level of liquid 26 and a level of proppant 30.

In stage 102 (shown in FIG. 9) level of liquid 26 is monitored in the mixture of proppant and liquid. Vessel 12 may be pressurized, for example above atmospheric pressure. The liquid may comprise LPG. Monitoring may further comprise monitoring the level of liquid 26 within a sheath, for example sheath 34, in the vessel 12, the sheath having at least an opening 40 at or near a lower end 38 of the sheath 34 for allowing liquid to flow into the sheath 34 and at least restricting the flow of proppant into the sheath 34, and an equalization vent 42. Monitoring may further comprise monitoring using TDR as described above for example. The liquid may be supplied into the vessel 12 from an inlet 22 in the vessel 12. The method may further comprise controlling the supply of liquid into the vessel 12 based on the monitored level of liquid 26 in the vessel 12.

In stage 104 (shown in FIG. 9), the mixture of proppant and liquid is supplied from the vessel 12 through an outlet 20 in the vessel 12. The mixture of proppant and liquid may be supplied from the vessel 12 to a pressurized stream of fluid, for example stream of fluid 24 illustrated in FIG. 6. The method may further comprise monitoring the level of proppant 30 in the vessel 12.

Referring to FIG. 1, vessel 12 may be mounted on a weight scale 58. Apparatus 10 may be used in combination with weight scale 58 in order to provide level of liquid 26 readings and weight readings of the contents of vessel 12. This way, signals may be sent as output from each respective device to a controller (not shown) that can then estimate the amount of proppant and the amount of liquid contained within vessel 12. This way, the controller will know when it is time to refill vessel 12 with proppant or re-supply liquid to interior 16. In some embodiments, the mixture of proppant and liquid may be supplied through inlet 18, which may be a hatch as shown.

Level of liquid 26 may not be the actual height of the liquid in the vessel. For example, when level 26 is below level 30, some liquid may be absorbed on to the proppant and may migrate to a higher level than in sheath 34. In this case, level 26 may be the level of liquid as discriminated by the discriminator 28. Referring to FIG. 8, an example of this is illustrated as level of liquid 26 is below the actual height of the liquid 32. Variations in the height of the liquid 32 and the level 26 may be the result of varying affinities between proppant and liquid, due to the types of proppant and liquid used. Thus, level monitor 14 may be used to directly measure the height of the liquid in the vessel 12, or it may be used to indirectly extrapolate the height of the liquid. This may be used to determine the amount of liquid in the vessel itself. In some embodiments, level monitor 14 may send as output signals indicative of either level 26 or the height of the liquid to, for example, a controller (not shown) that is part of the level monitor 14. The controller may control operation of the entire frac system (not shown), and may adjust the parameters of vessel 12 as needed. These methods give a good idea of the size of the liquid seal, and may be used to determine when and how much additional liquid to add to the proppant.

Liquefied petroleum gas (LPG) includes a variety of petroleum and natural gases existing in a liquid state at ambient temperatures and moderate pressures. In some cases, LPG refers to a mixture of such fluids. These mixes are generally more affordable and easier to obtain than any one individual LPG, since they are hard to separate and purify individually. Unlike conventional hydrocarbon based fracturing fluids, common LPGs are tightly fractionated products resulting in a high degree of purity and very predictable performance. Exemplary LPGs used in this document include ethane, propane, butane, pentane, hexane, and various mixes thereof. Further examples include i-butane, i-pentane, n-pentane, and n-butane. The LPG mixture may be controlled to gain the desired hydraulic fracturing and clean-up performance.

LPGs tend to produce excellent fracturing fluids. LPG is readily available, cost effective and is easily and safely handled on surface as a liquid under moderate pressure. LPG is completely compatible with formations and formation fluids, is highly soluble in formation hydrocarbons and eliminates phase trapping—resulting in increased well production. LPG may be readily and predictably viscosified to generate a fluid capable of efficient fracture creation and excellent proppant transport. After fracturing, LPG may be recovered very rapidly, allowing savings on clean up costs.

Referring to FIG. 3 an exemplary proppant addition system is illustrated. For exemplary purposes, the frac fluid source comprises LPG. LPG is supplied into suction manifold 60 through connections S1-S4 to create the stream of frac fluid. Additives, for example gelling agents, can be introduced into the stream of frac fluid via connections LA1-LA3. The stream passes through a flow conditioner 62 and a flowmeter 64 before passing a proppant introduction region 66, at which point proppant from vessels 12A and 12B may be introduced into the stream of frac fluid via augers 68A, 68B, respectively. The stream of frac fluid then continues along line 70 to discharge manifold 71, passing flowmeter 72 and nuclear densitometer 74. Densitometer 74 measures the wellhead density of the stream of frac fluid and proppant. Further additives may be added at this point via connections LA4-LA6. The stream of frac fluid can then be sent to a frac pumping system via connections D1-D4, where it will be pumped into a well to fracture a formation.

Nitrogen may be introduced into the system via lines 74, 76 from at least one N₂ pumper (not shown). Nitrogen pass through a surge tank 78, and can be supplied to at least one of the proppant vessels 12A, 12B, the LPG tanks, and all the lines that carry LPG at any point in the process. Nitrogen is supplied through a pressure regulator R1 to proppant vessels 12A and 12B via lines 80, 82, respectively. Nitrogen can then be supplied, via a series of valves to vessels 12A, 12B via connections at the top (84A, 84B) or bottom (86A, 86B) of the vessels. Nitrogen pressure may be controlled to add a suitable amount of pressure on the proppant and liquid in the vessels 12A, 12B. Nitrogen may also be sent to the LPG tanks via pressure regulator R2 to control the balance of pressure in the LPG tanks.

Proppant, for example sand, may be supplied to vessels 12A, 12B from a sand loader through lines 88A, 88B through sand inlets SI1-SI2. LPG can be controllably supplied to vessels 12A, 12B through connections S1-S4 of the suction manifold 60, and connection 90, which feeds to lines 90A, 90B. LPG can be added to the vessels 12A, 12B through a series of valves and via connections at the top (84A, 84B) or bottom (86A, 86B) of the vessels, similar to nitrogen addition. The pressure may be balanced and equalized between the LPG tanks and vessels 12A, 12B, in order to prevent over or under pressuring vessels 12A, 12B. Once the system is ready to add proppant to the stream of frac fluid, valves HV1 and HV2, which may be electro-hydraulic valves, are opened, and augers 68A and 68B supply the mixture of proppant and liquid into the stream of frac fluid at region 66. During the process, level monitoring devices 14C and 14D monitor the levels of liquid in vessels 12A, 12B, sending feedback to a controller in charge of the entire operation. Based on the levels of liquid in each respective vessel, the controller may decide to add more liquid to each respective tank as required.

The system may use relief valves and mechanisms at any point in the system as mandated for safety by law, for example relief valves RV1-RV5. Upon completion of the frac, connections D1-D4 may be closed, and all lines bled, for example via bleed valves BV1-BV3, through line 92, hose reel 94, and to the flare stack.

In the claims, the word “comprising” is used in its inclusive sense and does not exclude other elements being present. The indefinite article “a” before a claim feature does not exclude more than one of the feature being present. Each one of the individual features described here may be used in one or more embodiments and is not, by virtue only of being described here, to be construed as essential to all embodiments as defined by the claims. 

1. An apparatus for supplying proppant comprising: a vessel having an interior containing a mixture of proppant and liquid, an inlet for supplying proppant to the interior, and an outlet for supplying the mixture of proppant and liquid from the interior of the vessel; and a level monitor associated with the vessel for monitoring at least a level of liquid and having a discriminator for discriminating between the level of liquid and a level of proppant in the mixture of proppant and liquid.
 2. The apparatus of claim 1 in which the discriminator comprises a sheath having an upper end, a lower end, at least an opening at or near the lower end for allowing liquid in the interior to flow into the sheath and at least restricting the flow of proppant into the sheath, and an equalization vent.
 3. The apparatus of claim 2 in which the sheath is straight along a sheath length.
 4. The apparatus of claim 2 in which the sheath spans an upper end of the interior to a lower end of the interior.
 5. The apparatus of claim 2 in which the level monitor is configured to use time domain reflectometry to determine the level of liquid within the sheath.
 6. The apparatus of claim 2 in which the opening comprises at least one slot.
 7. The apparatus of claim 2 in which the opening comprises a proppant screen.
 8. The apparatus of claim 1 in which the level monitor is located at least partially outside the interior of the vessel.
 9. The apparatus of claim 1 in which the vessel further comprises a liquid inlet for supplying liquid to the interior of the vessel.
 10. The apparatus of claim 1 in which the vessel comprises a positive-pressure vessel.
 11. The apparatus of claim 10 in which the liquid comprises liquefied petroleum gas.
 12. The apparatus of claim 1 in which the outlet is connected to supply proppant into a pressurized stream of fluid.
 13. The apparatus of claim 1 in which the mixture of proppant and liquid is a mixture of proppant wetted with liquid.
 14. The apparatus of claim 1 in which the level monitor is configured to monitor both the level of liquid and the level of proppant.
 15. A method of supplying proppant comprising: providing a vessel at least partially filled with a mixture of proppant and liquid and having a level of liquid and a level of proppant; monitoring the level of liquid in the mixture of proppant and liquid; and supplying the mixture of proppant and liquid from the vessel through an outlet in the vessel.
 16. The method of claim 15 in which monitoring further comprises monitoring the level of liquid within a sheath in the vessel, the sheath having at least an opening at or near a lower end of the sheath for allowing liquid to flow into the sheath and at least restricting the flow of proppant into the sheath, and an equalization vent.
 17. The method of claim 16 in which the sheath spans from an upper end of the vessel to a lower end of the vessel.
 18. The method of claim 16 in which the opening comprises a proppant screen.
 19. The method of claim 16 in which monitoring further comprising monitoring using time domain reflectometry.
 20. The method of claim 15 in which the vessel is pressurized.
 21. The method of claim 20 in which the liquid comprises liquefied petroleum gas.
 22. The method of claim 20 in which the mixture of proppant and liquid is supplied from the vessel to a pressurized stream of fluid.
 23. The method of claim 15 in which liquid is supplied into the vessel from an inlet in the vessel.
 24. The method of claim 23 further comprising controlling the supply of liquid into the vessel based on the level of liquid in the vessel.
 25. The method of claim 15 in which the mixture of proppant and liquid is a mixture of proppant wetted with liquid.
 26. The method of claim 15 further comprising monitoring a level of proppant in the vessel.
 27. An apparatus for supplying proppant comprising: a pressure vessel having an interior containing a mixture of proppant and liquid, an inlet for supplying proppant to the interior, and an outlet at or near a base of the interior for supplying the mixture of proppant and liquid from the interior of the pressure vessel; and a level monitor associated with the pressure vessel for monitoring at least a level of liquid of the mixture of proppant and liquid.
 28. The apparatus of claim 27 in which the liquid comprises liquefied gas. 